Method and apparatus for controlling temperature of superheated steam



J. 28, w. L. DE BAUFRE 2,229,643

METHOD AND APPARATUS FOR GQNTRQLLING TEMPERATURE OF SUPERHEATED STEM!Filed Jan. 2, 1937 5 Sheets-She 1 PROM/6T6 aFT comaus T10 BURNER LEVELWe. 12 INVENTO/i Jan. 28, 1941.

W. L DE BA'UFRE METHOD AND APPARATUS FOR CONTROLLING TEMBERATURE QBSUPERHEATED STEAM Filed Jan. 2, 1937 3 Sheets-Shay 2 Jan. 28, 1941. w.L. DE BAUFRE 2,229,643

METHOD AND APPARATUS FOR CONTROLLING TEMPERATURE OF SUPERHEATED STEAMFiled Jan. 2, 1937 3 Sheets- Sheei 3 .27 ll llll ll I I l l alf ll llllIrrr nnn lllllll J'l g; Mprh 4 an I IIHIIIIIIIIIIII Ill IllllllllINVENTOR. W/LL/fl/W L. DEB/JUFAE. BY A A ATTORNEY.

Patented Jan. 28, 1941 METHOD AND APPARATUS FOR CONTROL- LING TEMPERATURSTEAM William Lane De Baufre,

E OF SUPERHEATED Lincoln, Nebn, assignor to The Superheater Company, NewYork, N. Y. Application January 2, 1937, Serial No. 118,906 13 Claims.(Cl. 12 2479) This invention relates to steam boilers and isparticularly applicable where the steam generated is superheated to ahigh temperature.

In some of the most modern steam power plants, for example, the steamgenerated is superheated to a temperature of 900 Fahrenheit. This highsuperheated steam temperature is desired in order to attain highefficiency of conversion of heat energy into mechanical work and also toreduce condensation of steam in the last stages of expansion in steamturbines. This high steam temperature cannot be safely exceededappreciably, however, even with the alloy steel used in the superheaterelements.

To reach this high superheated steam temperature but not exceed it,requires exceedingly careful proportioning of the heat absorbingsurfaces, both for generating steam and for superheating it. But even ifthe desired superheated steam temperature be just attained initially byvery careful designing, the superheated steam temperature will varyduring operation by reason of changes in cleanliness of the heatabsorbing surfaces. Thus, slag will form and adhere to the 25 heatabsorbing surfaces in the boiler furnace,

thereby reducing the effectiveness of such surfaces and raising thefurnace outlet temperature of the products of combustion. The furnaceoutlet temperature will also change with the percentage of excess airsupplied for combustion, with the characteristics of the fuel burned,and with the rate of combustion and the corresponding rate of steamgeneration. All these things will therefore affect the temperaturereached by the superheated steam, whether the superheating elements arelocated within the furnace where they absorb heat by radiation from theburning fuel and products of combustion. or whether they are locatedbeyond the furnace where they absorb heat by convection from theproducts of combustion after these products have left the furnace.

One object of the present invention is to attain almost exactly thesuperheated steam temperature desired without the necessity of socarefully proportioning the heat absorbing surfaces for generating steamand for superheating it.

Another object of the invention is to maintain a substantially constantsuperheated steam temperature with fluctuations in cleanliness of theheat absorbing surfaces, in the excess air supplied for combustion, inthe characteristics of the fuel burned, and in the rate of combustionand steam generation.

Another object of the invention is to maintain a high eiliciency ofcombustion and of steam generation while maintaining the superheatedsteam temperature substantially constant.

The foregoing together with such other advantages as hereinafter appearor are incident to the invention, are realized by the constructionillustrated in preferred form in the drawings, wherein Fig. 1 shows asteam boiler with convection and radiant superheaters in series arrangedto maintain a substantially constant su-. perheated steam temperature byrecirculating products of combustion, Fig. 2 illustrates theaccompanying variations in temperature of the burning fuel and productsof combustion, Fig. 3 shows a steam boiler with convection and radiantsuperheaters in reversed order to that in Fig. 1 arranged to maintain asubstantially constant superheated steam temperature by oxygenenrichment of the air supplied for combustion, Fig. 4 shows a steamboiler with radiant superheater 20 only and Fig. '5 shows a steam boilerwith convection superheater only, the latter two steam boilers beingarranged to maintain a substantially constant superheated steamtemperature by recirculating products of combustion. To illustrate dif-25 ferent types of firing, Fig. 1 and Fig. 2 indicate pulverized coalburners, Fig. 4 shows grates for hand firing and Fig. 5 indicates astoker. Fig. 6 is a sectional plan view on the line 6-6 in Fig. 1illustrating an arrangement in which the con- 30 vection superheaterpredominates, i. e. has more heat absorbing surface than the radiantsuperheater, while Fig. 7 is a similar view illustrating an arrangementin which the radiant superheater predominates, it being assumed that ineach fig- 35 ure the convection superheater is made up of looped tubes23 having a like number, say 4, of vertically spaced traverses as shownin Figs. 1 and 3.

Referring to Fig. 1 and Fig. 2, the steam and 0 water drum of the boileris shown at I. Water from drum 5 flows through downcomers 2 to header 3.From header 3, a portion of the water flows through screen tubes 4 toheader 5 and thence through wall tubes 6 to header i. Within 45 screentubes t and wall tubes 6, steam is generated due to heat absorption byradiation from the burning fuel and products of combustion within thefurnace. The commingled steam generated and remaining water flow throughtubes 8 to drum I.

Another portion of water flows from header 3 through wall tubes 9 tovertical headers in and thence through tubes H to vertical headers l2.Steam is generated within wall tubes 9 by radi- 55 ation from theburning fuel and products of combustion within the furnace. Steam isalso generated within tubes Ii partly by radiation of heat from thefurnace and partly by convection of heat from the products of combustionflowing between tubes II. The commingled steam generated and remainingwater flow through tubes l3 to drum l.

Water also flows from drum I through tubes H to vertical headers l5 andthence through tubes ii to vertical headers l1. Within tubes l6, steamis generated by convection of heat from the products of combustionflowing between tubes IS. The commingled steam generated and remainingwater flow through tubes l8 to drum l Within drum I, the steam generatedis separated from the remaining water, which is recycled. The saturatedsteam generated leaves through pipe i9. Feed water to maintain thedesired water level within drum l is supplied through pipe 20. The waterlevel within drum l is shown by means of gage glass and column 2 I.

In Fig. 1, the'saturated steam generated flows through pipe I9 to header22 and thence through tubes 23 to header 24. Within tubes 23, the steamis superheated by convection of heat from the products of combustionflowing between tubes 23. From header 24, the steam flows through pipe25 to header 26 and thence through tubes 21 to header 28. Within tubes21,.the steam is further superheated by radiation of heat from theburning fuel and products of combustion within the furnace. Thesuperheated steam finally leaves through pipe 29.

Instead of the steam being superheated first by convection and then byradiation, it may be superheated first by radiation and then byconvection by flowing through tubes 21 before flowing through tubes 23,as in Fig. 3, pipe l9 being connected to header 28 and pipe 29 to header22. This latter arrangement is preferable in certain cases in order toreduce the temperature of the metal in tubes 2'! which are exposed todirect radiation from the burning fuel and products of combustion withinthe furnace.

Instead of two superheaters in series, one superheater only may beemployed, as shown in Fig. 4 and Fig. 5. Then, in flowing through tubes21, Fig. 4, the steam will be superheated only by radiation of heat fromthe burning fuel and products of combustion within the furnace; or, inflowing through tubes 23, Fig. 5, the steam will be superheated only byconvection of heat from the products of combustion after they have leftthe furnace.

In Fig. 1 and Fig. 3, the furnace is bounded by screen tubes 4, walltubes 6, wall tubes 9, tube bank H and superheater tubes 21. Fuel in theform of pulverized coal and air for combustion are injected into thefurnace through burners 30 and Si. At the burner level, the burning fueland products of combustion reach a temperature of say 2600 Fahrenheit asindicated by curve A in Fig. 2 relative to the temperature scale shown.As the burning fuel and products of combustion swirl upwards through thefurnace and radiate heat to the surrounding heat absorbing surfaces, thetemperature drops as indicated by curve A. At the furnace outlet, thetemperature of the products of combustion is say 1900 Fahrenheit. Inflowing between tubes II, the temperature of the products of combustionis further reduced by convection heat transfer to say 1700 Fahrenheit,at which temperature the products of combustion enter the convectionsuperheater 23. Similar temperature conditions exist in the furnaces andboilers of Fig. 4 and Fig. 5.

For superheating the steam by convection of heat to tubes 23 beyond thefurnace, products of combustion are thus available at 1700 Fahrenheitafter the products of combustion have left the furnace. For superheatingthe steam by radiation of heat to tubes 21 within the furnace, burningfuel and products of combustion are available at temperatures reaching amaximum of 2600 Fahrenheit.

In Fig. 1 and Fig. 3, after giving up heat by convection to superheatertubes 23, the products of combustion iflow between tubes i6, I8, i3 and8, where the products of combustion are further cooled by convection ingenerating steam within these tubes. The products of combustion thenflow through air preheater 32 where these products are further cooled byheating the air for combustion. The products of combustion are finallydischarged by induced draft fan 33 through breeching 34 to a stack notshown. In Fig. 4 and Fig. 5, no air preheater nor induced draft fan isshown. In Fig. 4, the products of combustion flow through tubes 62 inthe return tu-, bular boiler 63.

In Fig. '1 and Fig. 3, air for combustion is supplied from duct 35 byforced draft fan 36 which forces the air through air preheater 32 andduct 31 to burners 30 and 3i. In Fig. 4, air for combustion is drawninto the ashpit under grates 60, while in Fig. 5, air for combustion isforced by forced draft fan under stoker 6i.

Induced draft fan 33 is driven by motor or turbine 38 and forced draftfan 38 is driven by a motor or turbine not shown in Fig. l but indicatedat 50 in Fig. 3.

In Fig. 1 breaching 34 is connected to duct 35 by duct 39 with damper 40arranged to vary the relative openings from ducts 35 and 39 for flow ofair and products of combustion respectively to forced draft fan 36. Whendamper 40 is moved up to close duct 39, air only will be forced into thefurnace by forced draft fan 36 and the temperature of the burning fueland products of combustion will vary as explained by reference to curveA in Fig. 2. When damper 40 is moved down to restrict the flow of airfor combustion through duct 35 and to permit products of combustion tobe drawn through duct 39 from breeching 34, these products of combustionwill be recirculated by being mixed with the air for combustion and thetemperature of the burning fuel and productsof combustion will then varyas indicated by curve B in Fig. 2 for the following reasons.

The presence of carbon dioxide and additional nitrogen commingled withthe air injected into the furnace with the fuel, reduces the maximumfurnace temperature. This reduction in temperature is due partly to heatabsorbed in dissociating the carbon dioxide but mainly to the additionalinert material to be heated by combustion. The heat absorbed indissociation later becomes sensible heat when the carbon dioxiderecombines as the temperature is reduced by radiation of heat from theburning fuel and products of combustion. Dilution likewise retardscombustion and lengthens the flames.

The amount of heat radiated from the burning fuel and products ofcombustion varies approximately as the fourth power of the absolutetemperature of the hot material. A small reduction in the maximumfurnace temperature therefore appreciably reduces the heat radiated athe burner level. But due to the larger am, unt of material present andto dissociation and later association when products .of combustion arerecirculated, the drop in temperature is less rapid to the furnaceoutlet than when no products are recirculated. In fact, with the maximumfurnace temperature reduced by recirculation from 2600" to say 2400Fahrenheit, the products of combustion may leave the furnace at 2000instead of 1900 Fahrenheit. With recirculation of products ofcombustion, the temperature of the burning fuel and products ofcombustion will therefore be represented by a curve such as B in Fig. 2and the products of combustion will reach convection superheater 23 atsay 1700 Fahrenheit as compared with 1600 Fahrenheit withoutrecirculation.

Therefore, in superheating steam by radiation of heat from burning fueland products of combustion within a furnace, the temperature of thesuperheated steam is reduced by recirculating more products ofcombustion due to a reduction. in the maximum temperature attained bythe burning fuel and products of combustion within the furnace. On theother hand, in superheating steam by convection of heat from products ofcombustion after they have left the furnace, the temperature of thesuperheated steam is increased by recirculating more products ofcombustion due to an increase in temperature of the products ofcombustion reaching the convection superheater.

When both radiant and convection superheating elements are employed inseries, the net effect will depend upon the relative amounts ofsuperheating acomplished in the two elements. If the radiant elementpredominates, as shown in Fig. 7, the maximum superheat temperature willdecrease with increase of recirculation. If the convection elementpredominates, as shown in Fig. 6, the maximum superheat temperature willincrease with increase in recirculation. For the purpose of illustratingthat the convection superheater predominates in Fig. 6 and the radiantsuperheater predominates in Fig. 7, it is to be assumed that theconvection tubes 23 are of looped form having a like number ofvertically spaced traverses in each figure, say 4, such traverses beingserially connected by return bends as illustrated in Fig. 1.

It is to be understood that recirculation of products of combustion forthis purpose is not excessive. With large amounts of productsrecirculated, the maximum superheat temperature with a convectionsuperheater will decrease instead of increase with increase inrecirculation I by reason of the diluting eifect of the productsrecirculated more than counteracting the effect of retardation ofcombustion.

Variation in the amount of products of combustion recirculated isaccomplished by varying the position of damper 40 which is moved by arm41 operated by mechanism 48. Mechanism 48 is automatically controlledbythe temperature of the superheated steam in pipe 29 through distant typethermometer 49. When the radiant type superheater 21 predominates,mechanism 48 moves arm 41 downwards to open damper 40 when thesuperheated steam temperature rises.

When the convection type superheater 23 predominates, mechanism 48 movesarm 41 upwards to close damper 40 when the superheated steam temperaturerises. That is, with both radiant superheater 21 and convectionsuperheater 23 in series, mechanism 48 is arranged to move arm 41 in thedirection corresponding to the superheater having the predominantefiect.

The use of air preheater 32 is desirable with recirculation of productsof combustion for controlling the superheated steam' temperature because the reduction in efliciency due to the products of combustionescaping from the steam generating surfaces at a slightly highertemperature with recirculation is compensated somewhat by the use of theair preheater. In an air preheater, conditions of heat transfer areadversely affected without recirculation due to-the fact that the air tobe heated is appreciably less in amount than the products of combustionto be cooled. With recirculation, with resulting improvement in heattransfer conditions. This improved heat transfer helps to maintain thehigh efficiency of steam generation as the amount of productsrecirculated is increased.

When products of combustion are recirculated, the draft loss due to flowof the products of combustion from the furnace over the heat absorbingsurfaces should not be maintained in proportion to the pressure drop ofthe steam generated flowing through a nozzle, as is customary whenproducts of combustion are not recirculated in order to maintain highcombustion efllciency. When products of combustion are recirculated, thedraft loss is not a measure of the air supplied for combustion. It istherefore proposed to compare the pressure drop of the air supplied forcombustion as it flows through duct 35 to forced draft fan 36 beforerecirculated products of combustion are mixed therewith, with thepressure drop of the steam generated as it flows througha nozzle 44 insteam pipe 29.

Flow meter 4| compares the rate of steam generated with the rate of airsupplied for combustion. Tubes 42 and 43 are connected before and afternozzle, or orifice, 44 in steam pipe 29. Tubes 45 and 46 are connectedto two points on duct 35, between which points an orifice may belocated. The two pens in fiow meter 4| draw curves on a revolving chartof air flow and steam flow. Asthe fuel fired is varied to maintain aconstant steam pressure with varying demand for steam from the boiler,forced draft fan 36 is varied in speed to maintain variations in the airflow curve on meter 4| corresponding with the variations in the steamflow curve.

Another way of regulating the superheated steam temperature by varyingthe relative amount of inert material mixed with the active fuel andoxygen, would be to inject more or less nearly pure oxygen with the airfor combustion and then vary the amount of oxygen so injected relativeto the air supplied for combustion as to maintain a substantiallyconstant superheated steam temperature, as indicated in Fig. 3 where 5|is a storage tank for nearly pure oxygen. The nearly pure oxygen flowsthrough'duct 52 and is drawn into duct 35 past damper 40 which isautomatically controlled by mechanism 48 arranged to move arm 41 in thedirection corresponding to the superheater 21 or 23 having thepredominant effect, so as to maintain the. superheated steam temperaturesubstantially constant in pipe 29. Recirculation of products ofcombustion is preferable, however, by reason of the cost of installingand operating a plant for producing oxygen. The increased efiiciency ofcombustion and steam generation by using oxythis ratio is nearer tounity.

for the additional cost of the oxygen.

A pulverized fu'el furnace has been illustrated in Fig. 1 forconvenience. The method and apparatus claimed, however, may be appliedto steam boilers fired with gas, oil or solid fuel or stokers or grateswith essentially the same results. In the boilers with stokers and withgrates in Fig. 5 and Fig. 4 respectively, an additional fan 63 is shownwith driving motor 54 because some such device would obviously berequired to recirculate the products of combustion, particularly with noforced draft fan in Fig. 4. This recirculating fan 53 is thenautomatically controlled by mechanism 65 operated by distant typethermometer 49 so as to maintain the superheated steam temperaturesubstantially constant in pipe 29. The steam pressure is customarilymaintained constant when fuel is flredon stokers or grates byautomatically controlling the supply of air for combustion. In Fig. 4,this is accomplished by damper 56 in duct 34 automatically operated bydamper controller 51 in order to maintain the steam pressuresubstantially constant in pipe 29 to which pressure tube 58 isconnected. In Fig. 5, it is accomplished automatically by the speed offorced draft blower 36 driven by motor 56 under the control of mechanism69 with pressure tube 56 to pipe 29. All

details required with pulverized fuel are not.

shown, such as the use of some of the preheated air for drying in themill and transporting the pulverized coal to the burners. These detailsare omitted as unessential to the invention. In Fig. 3, however, theautomatic control of fuel and air for combustion is indicated. Thus,some of the air for combustion is utilized to convey the pulverized coalthrough duct 60. Pulverized coal is fed to duct 60 by feeder 6| from bin62.

-This feeder is driven by motor 63 automatically controlled by mechanism64. Both mechanism 66 for motor 60 driving forced draft fan 36 andmechanism 64 for motor 63 driving pulverized feeder 6| are automaticallyoperated to maintain the steam pressure substantially constant in pipe29 by pressure tube 56 extending to mechanisms 69 and 64 from boilermeter ll. This arrangement is diagrammatic of control elements which arewell known in the art.

In the operation of a steam boiler with recirculation of products ofcombustion varied in accordance with my invention to maintain asubstantially constant superheated steam temperature, fuel and air forcombustion are supplied in the usual manner ,to maintain the desiredsteam pressure. Instead of using the draft loss of the products ofcombustion from the furnace to measure the supply of air for combustion,some more direct method is employed as described. Feed water is suppliedto maintain the desired water level within the steam drum. Thetemperature of the superheated steam is maintained substantiallyconstant by means of some device which automatically varies the amountof products of combustion recirculated until the superheated steamtemperature returns to the desired normal value when it deviatestherefrom.

Recirculation of products of combustion is not new; but it is believedto be new to utilize this means for controlling the temperature ofsuper-' heated steam in steam boilers. Recirculation of products ofcombustion in steam boilers has been proposed for recovering unconsumedfuel and returning it to the furnace for combustion.

But this is impractical because a furnace which is too small to burn thefuel completely without recirculation, is further overloaded when theunconsumed fuel is thus returned to the furnace.

In oil cracking stills, recirculation of products of combustion has beenemployed to lower the temperature of the products of combustion as theyleave the furnace, but the temperature within the furnace is unaffected.In the present proposal, the temperature of the products of combustionleaving the furnace is increased and the maximum furnace temperature isdecreased. Other proposed applications of recirculation of products ofcombustion to industrial furnaces differ still more from the presentproposal.

I claim:

1. The method of generating and superheating steam which includes thesteps of delivering fuel and air to a furnace, burning the fuel in thefurnace, and conveying away the products of combustion; transferringheat by radiation from the burning fuel and products of combustionwithin the furnace and by convection from the products of combustionafter they leave the furnace to water thereby evaporating it to steam;varying the rate of fuel and air delivery to the furnace to'maintain thepressure of the steam generated substantially constant; returning someof the products of combustion from a point beyond the furnace to thefurnace whereby the maximum temperature in the furnace is lowered, theamount of heat absorbed by radiation in the furnace reduced, and thetemperature of the gases leaving the furnace raised; transferring heatby convection from the products of combustion after they have left thefurnace to the steam to superheat it; and regulating the amount ofproducts of combustion returned to the furnace to keep the finaltemperature of the steam substantially constant.

2. The method of generating and superheating steam which includes thesteps of delivering fuel and air to a furnace, burning the fuel in thefur- ,nace, and conveying away the products of combustion; transferringheat by radiation from the burning fuel and products of combustionwithin the furnace and by convection from the products of combustionafter they leave the furnace to water thereby evaporating it to steam;varying the rate of fuel and air delivery to the furnace to maintain thepressure of the steam generated substantially constant; returning someof the products of combustion from a point beyond the furnace to thefurnace whereby the maximum temperature in the furnace is lowered, andthe amount of heat absorbed by radiation in the furnace reduced;transferring heat by radiation from the burning fuel and products ofcombustion in the furnace to the steam to superheat it; and regulatingthe amount of products of combustion returned to the furnace to keep thefinal temperature of the steam substantially constant.

3. The method of generating and superheating steam which includes thesteps of delivering fuel and air to a furnace, burning the fuel in thefurnace, and conveying away the products of combustion; transferringheat by radiation from the burning fuel and products of combustionwithin the furnace and by convection from the products of combustionafter they leave the furnace to water thereby evaporating it to steam;varying the rate of fuel and air delivery to the furnace to maintain thepressure of the steam generated substantially constant; returning someof the products of combustion from a point beyond the furnace to thefurnace whereby the maximum temperature in the furnace is lowered, theamount of heat absorbed by radiation in the furnace reduced, and thetemperature of the gases leaving the furnace raised; transferring heatboth by radiation from the fuming fuel and products of combustion in thefurnace and by convection from the products of combustion after theyhave left the furnace to the steam to superheat it; and regulating the.amount of products of combustion returned to the furnace to keep thefinal temperature of the steam substantially constant.

4. The method defined in claim 3, the greater part of the heat transferfor superheating the steam being by convection from the products ofcombustion after they have left the furnace.

5. The method as defined in claim 3, the greater part of the heattransfer for superheating the steam being by radiation from the burningfuel and products of combustion in the furnace.

6. The method defined in claim 1, said transfer of heat by convectionfrom the products of combustion after they have left the furnace tosuperheat the steam constituting at least the predominant part of theheat transferred to the steam to superheat it.

7. The method of generating and superheating steam which includes thesteps of delivering fuel and oxygen and inert material to a furnace,burning the fuel in the furnace and conveying away the products ofcombustion; transferring heat by radiation from the burning fuel andproducts of combustion in the furnace and by convection from theproducts of combustion after they leave the furnace to water therebyevaporating it to steam; varying the rate of fuel and oxygen deliveredto the furnace to maintain the pressure of the steam generatedsubstantially constant; transferring heat by convection from theproducts of combustion after they'have left the furnace to the steam tosuperheat'it; and maintaining the final temperature of the steamsubstantially constant by increasing the amount of inert materialrelative to the amount of oxygen as the temperature tends to fall,whereby the maximum temperature in the furnace is lowered the amount ofheat absorbed by radiation in the furnace reduced, and the temperatureof the gases leaving the furnace raised, and vice versa.

8. The method of generating and superheating steam which includes thesteps of delivering fuel and oxygen and atmospheric air to a furnace,

burning the fuel in the furnace and conveying away the products ofcombustion; transferring heat by radiation from the burning fuel andproducts of combustion in the furnace and by convection from theproducts of combustion after they leave the furnace to water therebyevaporating it to steam; varying the rate of fuel and oxygen andatmospheric air delivered to the furnace to maintain the pressure of thesteam generated substantially constant; transferring heat by convectionfrom the products of combustion after they have left the furnace to thesteam to superheat it; and maintaining the final temperature of thesteam substantially constant by increasing the amount of atmospheric airrelative to the amount of oxygen as said temperature tends to fall,whereby the. maximum temperature in the furnace is lowered, the amountof heat absorbed by radiation in the furnace reduced, and thetemperature of the gases leaving thefurnace raised, and vice versa.

9. The method of generating and superheating steam which includes thesteps of delivering fuel and oxygen and atmospheric air to a furnace,burning the fuel in the furnace and conveying away the products ofcombustion; transferring heat by radiation from the burning fuel and 5products of combustion in the furnace and by convection from theproducts of combustion after they leave the furnace to water therebyevaporating it to steam; varying therate of fuel and oxygen andatmospheric air delivered to the furnace to maintain the pressure of thesteam generated substantially constant; transferring heat by convectionfrom the products of combustion after they have left the furnace to thesteam to superheat it; and maintaining the final temperature of thesteam substantially constant by enriching the mixture of fuel, oxygen,and atmospheric air through increase in the amount of oxygen as saidtemperature tends to rise, where by the maximum temperature in thefurnace is lowered, the amount of heat absorbed by radiation in thefurnace reduced, and the temperature of the gases leaving the furnacelowered, and vice versa.

10. Apparatus for generating and superheating steam including a furnace,means for supplying fuel to said furnace, a forced draft fan forsupplying air to said furnace for burning said fuel, steam generatingsurface absorbing heat by radiation from the burning fuel and from theproducts of combustion in the furnace and heat by convection from theproducts of combustion beyond the furnace, means for maintaining the-maximum temperature in the furnace may be lowered and the heat absorbedbyrnadiation in the furnace be decreased so as to compensate for a risein the final temperature of the steam.

11. Apparatus in accordance with claim and further including a preheaterfor transferring heat from the products of combustion leaving thefurnace to the commingled combustion air and recirculated products ofcombustion on their way from the forced draft fan to the furnace.

12. Apparatus for generating and superheating steam including a furnace,means for supplying fuel to said furnace, a forced draft fan forsupplying air to said furnace for burning said fuel, steam generatingsurface absorbing heat by radiation from the burning fuel and from theproducts of combustion in the furnace and heat by convection from theproducts of combustion beyond the furnace, means for maintaining thepressure of the steam generated substantially constant by varying therate of combustion, heat absorbing surface for superheating the steamthrough transfer by convection from the products of combustion afterthey have left the furnace, an induced draft fan for withdrawingproducts of combustion from said furnace, means for recirculatingproducts of combustion from said induced draft fan to said forced draftfan, and means for regulating the means for recirculated products ofcombustion operable when final temperature of the steam falls toincrease the amount of products of combustion recirculated so that themaximum temperature in the furnace may be lowered, the heat absorbed byradiation in the furnace be decreased, and the temperature of the gasesleaving the furnace be raised so as to increase the final temperature ofthe steam.

13. The apparatus in accordance with claim 12 and further including apreheater for transferring heat for the products of combustion leavingto the furnace to the commingled combustion air and recirculatedproducts of combustion on their 5 way from the forced draft fan to thefurnace.

WILLIAM LANE DE BAUFRE.

